Analogs of cytochalasin B as radiopharmaceuticals for nuclear imaging of trans-membrane glucose transport

ABSTRACT

Analogs of cytochalasin B are used as radiopharmaceuticals to provide an image for the nuclear imaging of glucose transport across cell membranes. Like cytochalasin B, the analogs bind specifically to glucose transport protein, and in doing so, serve as markers indicating the amount of glucose transport protein present on the surfaces of cells. The analogs differ from cytochalasin B by the inclusion of a radionuclide in the region of the terminal phenyl group on cytochalasin B, where the radionuclide does not interfere with the binding specificity of the analog. Disclosed herein are such analogs as well as analogs with which readily accept radionuclides at the desired location on the structure, either by conjugation or substitution.

This invention lies in the field of nuclear medicine and ofradiopharmaceuticals used in nuclear imaging procedures.

BACKGROUND OF THE INVENTION

The rate of glucose metabolism by a living cell is known to be a usefulindicator of a variety of abnormal physiological conditions,particularly in human patients. Included among these conditions arevarious forms of cancer, coronary artery disease, brain tumors andepilepsy. The diagnosis and locale determination of these conditions hasbeen made possible by sophisticated imaging techniques that identifycells which are demonstrating abnormally high or low rates of glucoseintake.

Imaging up until now has been performed by positron-emission tomography(PET) with glucose analogs such as carbon-11-labeled glucose and ¹⁸F-labeled 2-deoxy-2-fluoro- D-glucose and its isomer ¹⁸ F-labeled3-deoxy-3-fluoro-D-glucose (the last two are referred to as "2-FDG" and"3-FDG," respectively). The analogs, upon administration to the patientprior to the imaging procedure, enter the cell in the same manner asglucose, and the resulting whole body distribution of the analogs asdetected by the imaging procedure indicates the stage and locus of theabnormality. These are the only known analogs which will achievetransport across the cell membrane in the same manner as glucose,because derivatization of the glucose molecule destroys the receptorspecificity of the molecule, thereby interfering with its transport. PETis the imaging technique of choice because it is sensitive enough tousefully detect the annihilation photons emitted by these analogs. Theadvantage of the FDG's over carbon-11-labeled glucose is that, unlikethe carbon-11-labeled glucose, the FDG's do not complete the metaboliccycle inside the cells, and therefore remain in the cells long enoughfor imaging to take place.

Unfortunately, PET is one of the more costly imaging procedures. As aresult, nuclear medicine scanning based on glucose transportabnormalities has enjoyed only limited use, and is feasible only atlocations where PET equipment is available. This has hindered thedevelopment of glucose transport both as a research tool and as adiagnostic method.

SUMMARY OF THE INVENTION

The present invention provides for nuclear imaging of glucose transportlevels based on the interaction of glucose transport protein andcytochalasin B. This invention differs from the prior art by associatingthe labeled species with the cell membrane rather than with the cellinterior. The labeled species in this case is an analog of cytochalasinB which bears a radiolabel in a location which does not interfere withthe interaction between cytochalasin B and glucose transport protein.

Glucose transport protein is any of five proteins present on themembranes of cells of the brain and other tissues in the body whichmetabolize glucose. These proteins are highly similar in structure andeach functions to facilitate the diffusion of glucose across the cellmembrane. The differences among these proteins are post-translationallyimposed, and are on portions of the proteins other than thoseresponsible for the glucose transport. In this specification, the term"glucose transport protein" is used to refer to any of the individualknown glucose transport proteins.

The amount of glucose transport protein present on the surface of a cellis changeable and varies with the cell's demand for glucose. A highdemand for glucose, for example, which is a characteristic of malignantcells, is responded to by a high concentration of glucose transportprotein on the cell membrane surface. Certain brain disorders, on theother hand, such as epilepsy and delayed development, are the result ofa defect or deficiency in the glucose transport protein.

Cytochalasin B is the most important and most biologically studied ofthe cytochalasins, which are a class of mold metabolites. Cytochalasin Bis obtainable from cultures of a Phoma sp. of mold, and for this reasonbears the generic name "phomin." The formula for cytochalasin B is asfollows: ##STR1##

A characteristic of cytochalasin B is that it specifically binds toglucose transport protein. The binding site is sufficiently removed fromthe phenyl ring, however, that variations of, and substitutions on, thephenyl ring do not affect the binding interaction between cytochalasin Band glucose transport protein. In accordance with the present invention,therefore, an imaging agent for nuclear imaging is provided byderivatizing a cytochalasin B molecule to contain a radionuclide on ornear the phenyl ring.

Since structural modifications in the region of the phenyl ring do notaffect the specific binding characteristics of the cytochalasin B,analogs bearing radioactive labels of many types can be prepared, all ofwhich will bind to glucose transport protein in the same manner ascytochalasin B. Radioimaging to detect the level and distribution ofglucose metabolism is thus no longer limited to PET, but can also bedone by nuclear scanning methods which require lower energy singlephoton emitters. Furthermore, unlike imaging methods which use an FDG asan imaging agent, imaging with these cytochalasin B analogs does notrely on the agent entering the cell, and is thus not in competitionwith, or otherwise affected by, the glucose which is also present in thetissue.

Accordingly, the invention resides in novel radiopharmaceuticals whichare cytochalasin B analogs. These analogs are identical to cytochalasinB except that the phenyl ring of cytochalasin B is either substitutedwith a radionuclide or replaced by a radionuclide or a group whichincludes a radionuclide. The invention further resides in stable(nonradioactive) cytochalasin B analogs which differ from cytochalasin Bin the same region as do the radiopharmaceuticals but which are readilyconvertible to the radiopharmaceuticals by simple chemical reactions.With these stable analogs as starting materials, the conversionreactions can be performed on site shortly before administration of theradiopharmaceuticals for imaging purposes. Still further, the inventionresides in methods for detecting and imaging levels of glucose transportprotein as a measure of the distribution of glucose metabolism in thebody of a patient.

Other features and advantages of the invention will be apparent from thedescription which follows.

DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS

Cytochalasin B analogs useful in the practice of the invention are thoseof the formula ##STR2## in which R is either: a radionuclide,

an aliphatic or alicyclic group containing a radionuclide,

a phenyl ring substituted with a radionuclide,

a phenyl ring substituted with a nonradioactive group which willfacilitate the substitution of a radionuclide at another location on thephenyl ring by a reaction such as a Bolton-Hunter reaction,

a phenyl ring substituted with a nonradioactive group which isreplaceable by a radionuclide through a simple chemical reaction such asan exchange reaction or a Sandmeyer reaction,

a conjugate of a radionuclide, or

a non-radioactive conjugating group which is capable of accepting aradionuclide by conjugation. Thus, the symbol R represents two classesof substituents--(1) those which are or contain radionuclides, in whichcase the analog is already a radiopharmaceutical and ready foradministration to a subject for an imaging procedure; and (2) thosewhich are nonradioactive precursors to which radionuclides are readilyjoined either by covalent bonds, electronic interaction or stericretention, either through conjugation or substitution. Examples ofreactions by which the nonradioactive analogs of the above formula canbe converted to radiopharmaceuticals are chelation reactions, exchangereactions, and electrophilic substitutions. For analogs which arethemselves radiopharmaceuticals, examples of R for radiopharmaceuticalsare:

monovalent radionuclides, aliphatic, alicyclic and aromatic groupscontaining monovalent radionuclide substituents,

aliphatic, alicyclic and aromatic groups containing polyvalentradionuclide substituents, and

radionuclide conjugates, including chelates of radionuclides and otherligand-bonded radionuclide structures.

Primary examples of aliphatic groups are alkyl groups such asstraight-chain C₁ -C₄ alkyls; primary examples of alicyclic groups aresaturated C₅ -C₇ cycloalkyls; and the primary example of an aromaticgroup is a phenyl group. Depending on its valence, the radionuclide mayeither be a substitute for a hydrogen atom or a link in an otherwisecarbon chain in the alkyl, alicyclic or aromatic structure. In preferredembodiments, R is either a radiohalogen-substituted phenyl group, achelate of a gamma-ray-emitting radionuclide, or a ligand-bondedradionuclide such as ligands known to be useful for technetium.

For embodiments in which R is a chelate, the chelating ligand may be anyof the wide variety of such ligands well known among those in nuclearmedicine and chelate chemistry. Examples of chelating ligands arediethylenetriamine pentaacetic acid (DTPA), ethylenediamine tetraaceticacid, nitrilotriacetic acid (NTA), and ethyleneglycolbis-bis(β-aminoethyl ether)-N,N-tetraacetic acid. Examples ofligands to which technetium can be conjugated are diamino dimercaptideand hydrazinonicotinamide. The derivatization and attachment of theseligands to the phenyl ring of the analog are matters of routinechemistry well known among those skilled in the art.

Examples of preferred atoms and groups for R in those embodiments inwhich the analog is a radiopharmaceutical are phenyl rings containing¹²¹ I, ¹²² I, ¹²³ I, ¹²⁴ I, 125I, ¹³¹ I, ⁷⁴ Br, ⁷⁵ Br, ⁷⁶ Br, ⁷⁷ Br,^(34m) Cl, ¹⁸ F or ²¹¹ At, and chelates of (or ligands bearing) ^(99m)Tc, ¹¹¹ In, ^(113m) In and ⁶⁷ Ga. Among these, particularly preferredare ¹²¹ I, ¹²² I, ¹²³ I, ¹²⁴ I, ¹²⁵ I, and ¹³¹ I. The most preferred is¹²³ I.

For those embodiments of the invention in which the analog is aprecursor to a radiopharmaceutical rather than a pharmaceutical itself,R is either a chelating ligand (which term is used herein to includetechnetiatable ligands), a phenyl ring substituted with an electrophilicactivating atom or group which activates the phenyl ring for acceptanceof a radionuclide, or a phenyl ring substituted with an atom or groupwhich is convertible by a simple chemical reaction or series ofreactions to a radionuclide or a group which contains a radionuclide.Examples of electrophilic activating groups are a hydroxyl group, anitro group, an amino group and an amido group. The term "amido" is usedherein to include formamido, acetamido, propionamido, and higheralkylated homologs. An example of a reaction by which a nonradioactiveanalog bearing an electrophilic activating group can be converted to aradiopharmaceutical is the Bolton-Hunter reaction, where theelectrophilic activating group permits substitution of a halogen at aposition either ortho- or para- to the activating group, using thehalogen in diatomic molecular form in the presence of a suitablesolvent. A particularly preferred electrophilic activating group in thiscategory is a hydroxyl group. For groups which are replaceable byradionuclides through simple chemical reactions, an example is the aminogroup, which can be oxidized to a diazonium group, then converted to ahalogen by a reaction such as a Sandmeyer reaction, a Gattermannreaction or a Korner-Contardi reaction. A further example of a groupreadily replaceable by a radionuclide is a stable halogen, which can besubstituted by a radioactive isotope of the same halogen by a simpleexchange reaction. A particularly preferred stable halogen for thispurpose is ¹²⁷ I, which can readily be replaced by ¹²³ I with anappropriate exchange reaction.

For nonradioactive analogs of this invention in which R is a phenyl ringbearing an electrophilic activating group, the preferred position of theelectrophilic activating group on the phenyl ring is the ortho-position.A prime example of R within this subclass is ortho-hydroxy phenyl. Fornonradioactive analogs in which R is a phenyl ring bearing a groupreplaceable by a radionuclide such as a radioactive halogen, thepreferred position of replaceable group on the phenyl ring is themeta-position. Prime examples of R within this subclass are meta-iodophenyl and meta-amino phenyl. Likewise, for radiopharmaceuticals withinthis invention in which R is a phenyl ring bearing a radionuclide, thepreferred position of the radionuclide on the phenyl ring is themeta-position.

This invention further extends to analogs in which R is a phenyl ringcontaining additional substituents other than the radionuclide,electrophilic activating group or replaceable group, which additionalsubstituents do not significantly detract from the utility of theanalogs as radiopharmaceuticals or radiopharmaceutical precursors.

The analogs of the present invention may be prepared from biologicallyderived cytochalasin B, with appropriate derivatization, or by synthesisfrom simpler starting materials. A synthesis of a typical analog withinthe scope of the invention, based on the syntheses of Stork, G., et al.,J. Am. Chem. Soc. 100(24):7775-7 (1978) and Masamune, S., et al., J. Am.Chem. Soc. 99:6756-8 (1977) is described in the succeeding paragraphs.In this synthesis, R is represented by a phenyl group bearing asubstituent X, which may be a radionuclide, an electrophilic activatinggroup, or an atom or group replaceable by a radionuclide. The symbol"AcO" denotes the acetate group. All reactions in this synthesis areconventional reactions for which detailed methods will be known to thoseskilled in the art of synthetic organic chemistry. Literature referencesare supplied in some cases as examples of sources in which thesereactions are described.

The starting material is the acetate of pure citronellol 1. This isconverted by treatment with O₃, CH₂ Cl₂, and Zn dust/acetic acid to thecrude aldehyde, which is then converted by a Jones oxidation to theacetate of 4(R)-methyl-6-hydrohexanoic acid 2. This product is thenreacted with the 1-ethyl ester of (+)-malic acid 3 by a Kolbeelectrolytic coupling reaction (as described by Horn, D. H. S., and Y.Y. Pretorius, J. Chem. Soc. 1460 (1954)), utilizing 0.2% ethanolicsodium ethoxide at about 1.5 A, at 45-50° C. for 75 minutes, followed bysolvent removal and reacetylation of the crude mixture. The couplingproduct, which is the diacetate of ethyl2-(R),8-dihydroxy-6(R)-methyloctanoate 4, is isolated in this reactionby elution. Reduction of the coupling product with LiAlH₄ in ether gives6(R)-methyl-2(R)-l,2,8-octanetriol 5, which is converted to thetriacetate 6 in silica gel, 4:1 hexane-ethyl acetate. The triacetate isconverted to the acetonide by reaction with acetone andp-toluenesulfonic acid at room temperature for 3 hours, and theacetonide is converted by Collins oxidation for 1 hour at roomtemperature to the aldehyde 7. ##STR3##

A trans,trans-dienic compound, for coupling with the aldehyde 7, isprepared as follows.

Glycidaldehyde 8 is condensed with carboxymethylene triphenylphosphorane(30% excess in benzene, 1.5-hour reflux) to give the unsaturated ester9. The ester is then treated with formic acid for thirty minutes,followed by concentration under vacuum, overnight treatment with aqueoussaturated bicarbonate, washing in hexane, and ethyl acetate extraction,to form the glycol 10. The primary alcohol on 10 is then protected asthe tert-butyl dimethylsilyl ether and, after separation from somedisilylated compound, is oxidized by CrO₃ •2C₅ H₅ N to the unsaturatedketo ester 11. The ester is then condensed withethylidenetriphenylphosphorane in tetrahydrofuran at -78° C. for 45minutes, to give a 5.7:1 ratio of the desired trans,trans-dienic ester12 and its trans,cis-isomer, with separation of the two readilyaccomplished by elution with 3% ethyl acetate in hexane, since thetrans,cis-isomer is more easily eluted. Reduction of thetrans,trans-dienic ester 12 with sodium bis(methoxyethoxy)aluminumhydride gives the corresponding alcohol 13. The alcohol is thendissolved in 3:1 ether-hexamethylphosphoramide and treated withbutyllithium at -30° C., followed by p-toluenesulfonyl chloride, and,after thirty minutes, by sodium diethyl phosphite in toluene, to formthe dienyl phosphonate 14. ##STR4##

The dienyl phosphonate 14 is converted to its sodium salt by reactionwith sodium hydride in benzene, with 0.25 equivalents of methanol fortwo hours at 55° C., and then condensed with the aldehyde 7 at 55° C.overnight, to form the desired triene 15: ##STR5##

A pyrrolone to act as a dienophile for cycloaddition to the triene 15 isprepared as follows. Methyl L-3-amino-(substituted phenyl)butyrate 16 isprepared from the appropriately substituted N-carbobenzoxy-L-alanine byArndt-Eistert homologation, via the silver oxide-methanol rearrangementof the diazo ketone, followed by 10% Pd/C hydrogenolysis. The aminoester 16 is converted into the hydroxypyrrolone ester 17 by the methodof Southwick, P. L., and R. T. Crouch, J. Am. Chem. Soc. 75:3413 (1953).The hydroxypyrrolone ester 17 is then treated with aceticanhydride-pyridine, with 4-N,N-dimethylaminopyridine as catalyst, forthirty minutes at room temperature, followed byhydrolysis-decarboxylation, using dimethyl sulfoxide-sodiumchloride-water (50:28: 1), at 135°-140° C. for 1.5 hours in a nitrogenatmosphere. The liberated hydroxyl is then reacetylated, and theN-acetyl pyrrolone acetate 18 is isolated by chromatography on silica,using 4:1 hexane-ethyl acetate followed by 3:2 chloroform-ethyl acetate.##STR6##

Cycloaddition of the triene 15 to the N-acetyl pyrrolone acetate 18 isperformed in xylene at 170° C. over 4 days, to form the adduct 19. Thesilyl protecting group of the adduct 19 is then removed by treatmentwith 3:1:1 acetic acid-water-tetrahydrofuran, at room temperatureovernight, followed by acetone-p-toluenesulfonic acid for 1.5 hours atroom temperature, to yield the unprotected compound 20. The unprotectedcompound is then treated with tert-butyl hydroperoxide and Mo(CO)₆,under reflux in benzene for 1.5 hours, to yield the epoxide 21, which isthen treated with carbon tetrabromide and triphenylphosphine for 4 hoursat room temperature, to yield the bromide 22, followed by β-eliminationusing zinc dust/sodium iodide to give the2-hydroxy-1-methylenecyclohexane compound 23. ##STR7##

To convert the terminal isopropylidene group of the2-hydroxy-1-methylene-cyclohexane compound 23 to a 4-hydroxytrans-a,β-unsaturated ester system, the terminal isopropylidene group isreacted with aqueous acetic acid-tetrahydrofuran for 4 hours at roomtemperature to liberate the 1,2-glycol, leaving the primary alcohol 24.The primary alcohol is then protected as the tert-butyl dimethylsilylether 25, and the two secondary hydroxyls are protected as thetetrahydropyranyl ethers 26. The primary alcohol 27 is then liberated bytreatment with tetrabutylammonium fluoride, followed by oxidation to thealdehyde 28 using a Collins reaction, and condensation withmethyltriphenylphosphoranylidene acetate to yield the 4-hydroxytrans-α,β-unsaturated ester 29. The ester is then treated with 1 Nethanolic sodium hydroxide at 60° C. for 1 hour, followed bychromatography on silica with 9:1 CHCl₃ --CH₃ OH to yield the hydroxyunsaturated acid 30. The hydroxy unsaturated acid is subsequentlycyclized by being heated with Na₂ HPO₄ and AgCF₃ CO₂ in benzene atreflux under an argon atmosphere, followed by mild alkaline hydrolysis.The product is the cytochalasin B analog of the present invention.##STR8##

The placement of X on the phenyl ring of the N-carbobenzoxy-L-alanineused in the preparation of the methyl L-3-amino-(substituted phenyl)butyrate 16 is achieved by conventional methods. In preparingnonradioactive precursors to the radiopharmaceuticals, it is preferredto begin the synthesis with an N-carbobenzoxy-L-alanine already bearingthe X substituent. Conversion to a radiopharmaceutical is then performedon the otherwise fully synthesized analog. To place radioactive halogenson the ring, for example, a variety of methods can be used. Startingwith a nonradioactive analog bearing an amine group on the phenyl ring,the amine group can be oxidized to a diazonium group, which is thenreplaced by a radioactive halogen such as fluorine either in thepresence of cuprous salts (Sandmeyer reaction), copper powder(Gattermann reaction) or cupric salts (Korner-Contardi reaction).Radiopharmaceuticals bearing radioactive bromine can be derived from anonradioactive analog bearing an ortho-hydroxy group as X, by treatingthe analog with molecular bromine in the presence of acetic acid, Femetal, carbon disulfide or t-butylamine, using a Bolton-Hunter reaction.With an ortho-hydroxy group as X, substitution of the halogen at themeta-position can be achieved by a Bolton-Hunter reaction, using amolecular halogen and either carbon disulfide or tributylamine andtoluene at low temperatures. Nonradioactive analogs bearing stablehalogens as X can be converted to radiopharmaceuticals by substitutingthe stable halogen with a radioactive halogen, using an exchangereaction according to well-known techniques.

The preparation of analogs in which R is a chelate or other conjugatingligand rather than a substituted phenyl ring is achieved by using theligand substituted with methyl L-3-aminobutyrate in place of the methylL-3-amino-(substituted phenyl)butyrate 16. Methods of preparing ligandssubstituted in this manner are known among those skilled in syntheticchemistry.

The conversion of a nonradioactive analog within the scope of thisinvention to a radiopharmaceutical can be performed by the user shortlyprior to administration to a patient, or by a central processingfacility located in close proximity to a diagnosis laboratory in ahospital or clinic. Other arrangements for forming and using thecompounds will be readily apparent to clinicians, technicians anddoctors who are experienced in imaging procedures and techniques.

Administration of the cytochalasin B analogs of the present inventionfor purposes of nuclear imaging is achieved by conventional procedures.Aqueous solutions of the analogs, and in particular, solutions inphysiological saline, are most conveniently used. The concentrations ofthe analogs in these solutions and the amounts administered may varywidely, the optimum in each case varying with the strength of theradionuclide in the analog, the method of administration, the degree ofactivity uptake desired or needed, and the age, weight and condition ofthe patient or subject to whom administration is made. A typical dosefor an adult human is approximately 2.5 milligrams of theradiopharmaceutical in physiological saline at a concentration of 5 μM,or approximately 35 μg per kg of the subject's whole body weight. Interms of the radionuclide such as ¹²³ I, a typical dose would be 5-10millicuries. Administration may be achieved by any parenteral route andmethod, most notably by intravenous administration.

The cytochalasin B analogs of this invention are useful in a wide rangeof imaging techniques. Examples are conventional diagnostic gamma-raycamera imaging and more complex techniques such as single photonemission computed tomography (SPECT) and positron emission tomography(PET).

The foregoing is offered primarily for purposes of illustration. It willbe readily apparent to those skilled in the art that the descriptions ofthe preferred pharmaceutical agents and administration methods describedherein may be further modified in various ways without departing fromthe spirit and scope of the invention.

What is claimed is:
 1. A compound having the formula ##STR9## in which Ris a member selected from the group consisting of: a radionuclide,analiphatic or alicyclic group containing a radionuclide, a phenyl ringsubstituted with a radionuclide, and a chelate of a radionuclidesaidradionuclide being selected from the group consisting of ¹²¹ I, ¹²² I,¹²³ I, ¹²⁴ I, ¹²⁵ I, ¹³¹ I, ⁷⁴ Br, ⁷⁵ Br, ⁷⁶ Br, ⁷⁷ Br, ^(34m) CL, ¹⁸ F,²¹¹ At, ^(99m) Tc, ¹¹¹ In, ^(113m) In and ⁶⁷ Ga.
 2. A compound inaccordance with claim 1 in which R is a member selected from the groupconsisting of:phenyl rings containing monovalent radionuclidesubstituents, and chelates of a radionuclide.
 3. A compound inaccordance with claim 1 in which R is a member selected from the groupconsisting of:a phenyl ring containing a substituent which is a memberselected from the group consisting of ¹²¹ I, ¹²² I, ¹²³ I, ¹²⁴ I, ¹²⁵ I,¹³¹ I, ⁷⁴ Br, ⁷⁵ Br, ⁷⁶ Br, ⁷⁷ Br, ^(34m) Cl, ¹⁸ F and ²¹¹ At, and achelate of a member selected from the group consisting of ^(99m) Tc, ¹¹¹In, ^(113m) In and ⁶⁷ Ga.
 4. A compound in accordance with claim 1 inwhich R is a member selected from the group consisting of:a phenyl ringsubstituted in the meta-position with a member selected from the groupconsisting of ¹²¹ I, ¹²² I, ¹²³ I, ¹²⁴ I, ¹²⁵ I, ¹³¹ I, ⁷⁴ Br, ⁷⁵ Br, ⁷⁶Br, ⁷⁷ Br, ^(34m) Cl, ¹⁸ F and ²¹¹ At, a chelate of a member selectedfrom the group consisting of ¹¹¹ In, ^(113m) In and ⁶⁷ Ga, in which thechelating ligand is a member selected from the group consisting ofdiethylenetriamine pentaacetic acid, ethylenediamine tetraacetic acid,nitrilotriacetic acid, and ethylene glycol-bis(β-aminoethyl ether)-N, N-tetraacetic acid, technetium diamino dimercaptide, and technetiumhydrazinonicotinamide.
 5. A compound in accordance with claim 1 in whichR is a member selected from the group consisting of:a phenyl ringsubstituted in the meta-position with a member selected from the groupconsisting of ¹²¹ I, ¹²² I, ¹²³ I, ¹²⁴ I, ¹²⁵ I, and ¹³¹ I, a chelate ofa member selected from the group consisting of ¹¹¹ In, ^(113m) In and ⁶⁷Ga, in which the chelating ligand is a member selected from the groupconsisting of diethylenetriamine pentaacetic acid and nitrilotriaceticacid, technetium diamino dimercaptide, and technetiumhydrazinonicotinamide.
 6. A compound in accordance with claim 1 in whichR is a phenyl ring substituted in the meta-position with a memberselected from the group consisting of ¹²¹ I, ¹²² I, ¹²³ I, ¹²⁴ I, ¹²⁵ I,and ¹³¹ I.
 7. A compound in accordance with claim 1 in which R is aphenyl ring substituted in the meta-position with ¹²³ I.